Data processors are frequently used in two types of operating applications. A first operating application is a use in a high-performance, zero-wait state system. In a high-performance system, output buffers which have a very fast slew rate are used at output terminals. In a printed circuit board to which data processors are commonly connected, bus lines are terminated to a reference potential to control conventional ringing effects. High-performance terminated lines require interfacing with an output buffer having a low output impedance, such as ten ohms or less. Output buffers having such low impedance require transistors with large physical dimensions in order to achieve proper logic levels. A terminated line has the disadvantages of additional components and circuit area and increased power consumption. A second operating application is a use in a mid-performance, lower cost system. In the mid-performance system, output buffers with slower slew rates may be implemented and may be used with non-terminated lines on a printed circuit board. An output buffer for a non-terminated line typically requires a higher impedance to avoid conventional line ringing. Data processors which use output buffers for non-terminated line applications are less costly to implement in part because transistors having smaller dimensions may be used.
Data processors have typically tried to satisfy both types of output impedance requirements in order to be satisfactory for either high-performance or mid-performance applications. In order to be applicable for both terminated line and non-terminated line applications, a data processor having an output driver circuit with a fixed output terminal impedance somewhere between low and high impedance application values has been implemented. Examples of such data processors are the commercially available MC68020 and the MC68030 data processors offered by Motorola Inc.